1. Field of the Invention
The present invention relates to optical communications systems and, more particularly, to optical amplifiers.
2. Description of the Related Art
As the result of continuous advances in technology, particularly in the area of networking such as the Internet, there is an increasing demand for communications bandwidth. For example, the transmission of data over a telephone company's trunk lines, the transmission of images or video over the Internet, the transfer of large amounts of data as might be required in transaction processing, or videoconferencing implemented over a public telephone network typically require the high speed transmission of large amounts of data. As applications such as these become more prevalent, the demand for communications bandwidth capacity will only increase.
Optical fiber is a transmission medium that is well suited to meet this increasing demand. Optical fiber has an inherent bandwidth that is much greater than metal-based conductors, such as twisted pair or coaxial cable; and protocols such as the OC protocol have been developed for the transmission of data over optical fibers. Typical communications system based on optical fibers include a transmitter, an optical fiber, and a receiver. The transmitter converts the data to be communicated into an optical form and then transmits the resulting optical signal via the optical fiber to the receiver. The receiver recovers the original data from the received optical signal.
Optical amplifiers, which boost the power of the optical signal propagating through the optical fiber, are an important component in such fiber communications systems. For example, receivers typically operate properly only within a relatively narrow range of optical signal power levels; optical amplifiers can be used to boost the received optical signal to the proper power range for the receiver. As another example, phenomena such as fiber losses, losses due to insertion of components in the transmission path, and splitting of the optical signal may attenuate the optical signal and degrade the corresponding signal-to-noise ratio as the optical signal propagates through the communications system. Optical amplifiers may be used to compensate for these attenuations. Conventional optical amplifiers, however, suffer from various drawbacks.
Fiber amplifiers are one type of conventional optical amplifier. They include a length of fiber which has been doped to form an active gain medium. Ions of rare-earth metals, such as erbium, are typically used as the dopant. The doped fiber is typically pumped by an optical pump at a wavelength which is preferentially absorbed by the ions but different from the wavelength of the optical signal to be amplified. The pumping results in a population inversion of electronic carriers in the active medium. Then, as the optical signal propagates through the doped fiber, it is amplified due to stimulated emission.
One drawback of fiber amplifiers is that they typically can only operate over a narrow wavelength range when multiple fiber amplifiers are cascaded. This is especially problematic if the optical signal to be amplified covers a wide range of wavelengths, as would be the case if the entire bandwidth of the optical fiber were to be efficiently utilized. Another disadvantage of fiber amplifiers is their transient response to channel drop-out in wavelength division multiplexing systems. Further problems with fiber amplifiers include their relatively large size, slow switching speed, power inefficiency, difficulties in mass producing them, and their high cost which makes them prohibitively expensive for many applications.
Non-lasing semiconductor optical amplifiers (SOAs) are an alternative to fiber amplifiers. Non-lasing semiconductor optical amplifiers are typically based on a semiconductor laser-like structure which is operated below the lasing threshold. Typically, an electrical current pumps the active region of the amplifier, resulting in an increased carrier population. The optical signal then experiences gain as it propagates through the active region due to stimulated emission.
One problem with non-lasing semiconductor optical amplifiers is that the gain depends on the amplitude of the optical signal. For example, a strong optical signal will be amplified less than a weak signal and strong portions of the optical signal will be amplified less than weak portions. This results in distortion of the optical signal and possibly also crosstalk between different optical signals propagating simultaneously through the system. This problem is the result of gain saturation, in which there are insufficient carriers in the conduction band to provide the full amount of gain to higher power signals.
Lasing semiconductor optical amplifiers can overcome the problem of gain saturation. These amplifiers are also based on a semiconductor active region. However, the active region is pumped above the lasing threshold. The gain is then clamped due to the lasing action and is fairly constant until the amplifier reaches its power limit.
However, lasing semiconductor optical amplifiers also suffer from inherent drawbacks. For example, there is an inherent tradeoff between noise performance and power output. If the carrier density at the lasing threshold is high, the amplifier will have good noise performance but will have a low saturable power thus limiting its power output. On the other hand, an amplifier with a low carrier density at the lasing threshold will be capable of large power output but suffer from poor noise performance. This inherent tradeoff makes it difficult for a lasing semiconductor optical amplifier to attain both a low noise and a high power output.
Lasing semiconductor optical amplifiers are also limited in that the gain of the amplifier typically cannot be adjusted in order to accommodate different optical signals that propagate through the amplifier. For example, optical receivers typically operate properly only within a relatively narrow range of optical signal power levels. The ability to adjust or tune the gain of an optical amplifier would be beneficial because it would allow the amplifier to dynamically provide the appropriate gain to the optical signal so that it falls within the range of the optical receivers.
Thus, there is a need for an optical amplifier which does not suffer from gain saturation and is also capable of both low noise and high power output. In addition, there is also a need for an optical amplifier whose gain can be adjusted or tuned during operation of the optical amplifier.